The present invention relates to a calibration method and apparatus for calibrating the location and orientation of sensors with respect to the beam plane (scanning plane) of ultrasound scanning transducers. The invention is particularly useful for calibrating position sensors on ultrasound scanning transducers used in medical applications, e.g., for the purpose of guiding insertion devices, such as aspiration and biopsy needles, endoscopes, etc., through body tissue, and is therefore described below with respect to such applications. The invention also relates to calibration of any other component of a position measuring system being attached to an ultrasound transducer or other imaging device, in position measuring systems, that are for example, acoustic, magnetic or optic.
In the foregoing medical systems for guiding, e.g., a biopsy needle to a target in body tissue, the part of the body in which the tissue is located is usually imaged by an ultrasound transducer which scans the body along scan planes. The location and orientation of a rigid straight needle, or other insertion device, is determined by a position sensor secured at a predetermined location on the needle. The absolute location and orientation of the plane displayed by the imaging system must also be determined, this being done by a position sensor secured at a given convenient location on the ultrasound scanning transducer. The system enables the measurement of the relative location and orientation of the needle with respect to the target tissue also to be calculated. Once these values are determined, it is possible to compute the expected path of the needle towards the target and to display it on the image in order to enable the physician to navigate the needle precisely towards the target
An example of such an imaging system is described in our Patent Application No. PCT/IL96/00050 published Feb. 6, 1997, which is hereby incorporated by reference.
In such a system, the position sensor, secured to a predetermined point on the needle, measures the precise location and orientation of the needle upper tip but the position sensor, being attached to the ultrasound transducer at a convenient, arbitrary location thereon, does not have a well determined spatial position and orientation to the scan plane of the transducer so as to precisely relate the transducer position sensor to the transducer scan plane. Yet, since the navigation of the needle to the target uses the ultrasound image as a background for the display of the future path of the needle, it is imperative to calculate the precise location and orientation of the scan plane with respect to the position sensor on the ultrasound transducer.
A method described in relation to calibrating a magnetic position sensor being affixed to an ultrasound transducer is described in, Detmer et al., in xe2x80x9c3D Ultrasonic Image Feature Localization Based on Magnetic Scanhead Tracking. In Vitro Calibration and Validation. This method has the drawback that the ultrasound transducer must be maneuvered at a relatively large number of positions, and complicated mathematical algorithms are employed in order to resolve ultrasound beam plane orientation with respect to the target. The methods presented herein alleviate these aforementioned drawbacks, by utilizing ultrasound target(s) that are known and fixed spatially with respect to a position measuring component.
An object of the present invention is to provide a method and apparatus for calibrating a scanning transducer, particularly an ultrasound scanning transducer (ultrasound transducer), in order to calculate the precise location and orientation of its scan plane with respect to the position sensor on the ultrasound transducer.
According to one aspect of the present invention, there is provided a method of calibrating a transducer, outputting a scanning beam for scanning a body along a scanning plane through the body, which transducer has a first position sensor attached to it at a fixed location thereon for measuring the location and orientation of the transducer position sensor with respect to the scanning plane, comprising the operations:
(a) actuating the transducer to scan a body or body volume along a scanning plane or volume while measuring the location and/or the orientation of the position sensor with respect to a fixed reference point in space,
(b) utilizing a second position sensor to measure the location and orientation of the scanning plane with respect to the reference point in space while the transducer scans the body along the scanning plane,
(c) and computing, from the measurements produced in operations (a) and (b), a value representing the location and orientation of the transducer scanning plane with respect to the first position sensor for use in calibrating the first position sensor according to the position at which it is fixed on the transducer.
According to additional aspects of the present invention, the second position sensor can be replaced by a transmitter and the position of the first position sensor, being attached to the ultrasound transducer, is calculated with respect to this transmitter
According to additional aspects of the present invention, the position sensor affixed onto the ultrasound transducer is some other component of a position measuring system (transmitter/reflector) and the second position sensor is some other component of the position measuring system; such that the relative position between the above two position system measuring components can be measured directly or through measurement relative to a reference location in space, in accordance with the systems detailed in PCT/IL96/00050 and PCT/IL98/00578, PCT/IL98/000578 also incorporated by reference in its entirety herein.
Examples of the invention are now described below.
In one described embodiment, operations (a) and (b) are performed by using a calibration device defining a reference plane, which calibration device includes the second position sensor (or other suitable position measuring component, for example transmitters, receivers, reflectors, transceivers, etc), and by aligning the scanning plane of the transducer with the reference plane in the calibration device.
In the second described embodiment, the second position sensor (or other suitable position measuring component) is located, when operation (b) is performed, on a device which is manipulated to sense at least three non-collinear points defining the transducer scanning plane, and to measure the location and orientation of the three points with respect to the fixed reference point in space. For example, where the device is a medical device such as a biopsy needle, the second position sensor would be at known location on the biopsy needle with respect to its tip.
According to a further aspect of the present invention, there is provided apparatus for calibrating a transducer in accordance with foregoing methods.
According to a still aspect of the present invention particularly useful in the first embodiment, there is provided a calibration device for calibrating a position sensor on and with respect to an ultrasound transducer used for scanning a body along scanning planes through the body, comprising:
a charmer filled with a liquid having ultrasonic propagation properties similar to those of the body to be scanned;
a wall of the chamber being formed with an opening covered by an acoustical matched membrane adapted to receive the ultrasonic transducer having the position sensor to be calibrated;
and a plurality of at least three non-collinear echogenic elements located within the chamber at predetermined locations therein so as to define a reference plane within the chamber such as to enable the location and orientation of the position sensor to be calculated with respect to the scan plane of the ultrasound transducer.
According to further features in the latter described embodiment, the chamber is in the configuration of a right rectangular prism, including first and second lateral walls, first and second end walls, a top wall, and a bottom wall. The plurality of echogenic elements include first and second groups of small cross-section rods of echogenic material secured to said first and second lateral walls, respectively, and extending perpendicularly thereto inwardly of the chamber, the rods of each group being non-aligned with respect to the rods of the other group and having free tips located at said reference plane within the chamber.
According to other aspects of the present invention, particularly useful in the first embodiment, the chamber can have any form that does not introduce image distortion to the ultrasound image, and/or the echogenic targets are lines or polygons or other shapes, that can be arranged in a manner that defines a reference plane.
As will be described more particularly below, the calibration method and apparatus of the present invention enable transducers to be calibrated so as to precisely determine the location and orientation of the scan plane with respect to the position sensor (or other position measuring component) on the transducer. Once this has been determined with respect to a position sensor secured on the transducer, this calibration data remains the same for the respective transducer until the location of the position sensor on the transducer is changed, whereupon the transducer must be recalibrated.
The present invention can be used for calibrating any component of a position measuring component being attached to an ultrasound transducer. In particular, the present invention can be used for calibrating any component of a position measuring component being attached to an ultrasound transducer, in conjunction with the guiding systems disclosed in PCT/IL96/00050 and PCT/IL98/00578.
Further features and advantages of the invention will be apparent from the description below.